The present invention relates generally to the measurement of weak optical absorptions and more particularly to the use of the thermooptic effect which occurs when a laser beam traverses a weakly absorbing solution forming a negative thermal lens which lens, when automatically correcting for a deliberate optical misalignment of a laser cavity into which the sample under investigation is inserted, causes pulsed operation in what would normally be continuous laser output, the pulsewidth of this output being related to the sample absorptivity.
When a laser beam traverses a weakly absorbing solution a negative thermal lens is formed in the solution due to heating of the liquid. This is one manifestation of the thermooptic effect, and it can be used to measure small absorptivities, .alpha., of solutions or molar extinction coefficient of solutes, .epsilon..sub.s =.alpha..sub.s /c, where .alpha..sub.s is the absorptivity of the solute and c is the solute concentration in a particular solvent. A cell containing the solution to be studied is inserted into the cavity of a normally continuous-wave dye laser. If the plane of the output coupler of the laser is intentionally misaligned slightly, the laser begins pulsed operation at a frequency essentially independent of .alpha. and only slightly dependent on the extent of the cavity misalignment. The pulses are equally spaced and have identical pulsewidths which width is strongly dependent on the absorptivity of the sample solution. Such pulsewidths may decrease by as much as a factor of 100 as the absorptivity is increased making this characteristic useful for measuring weak absorptivities of solutions.
The thermooptic phenomenon has been known for many years, and has been used for almost as many years for absorption measurements. Reference 1 describes transient effects in lasers with liquid samples inserted in the laser cavity, but the method and apparatus of the instant invention utilizes a different manifestation of the thermooptic effect from other previous absorption measurement techniques or this transient-based method.
1. In "Long-Transient Effects in Lasers with Inserted Liquid Samples," by J. P. Gordon, R. C. C. Leite, R. S. Moore, S. P. S. Porto and J. R. Whinnery, J. Appl. Phys. 36, 3 (1965), the buildup and decay of the output of a helium-neon laser when absorbing liquids were placed within its resonator cavity is described. The application of the authors' method to measurement of small absorbances is mentioned although not elaborated upon. The transient buildup and decay of the laser output takes place over a timescale of several seconds and appears to be explainable by the simple heating of the liquid with the formation of a thermal lens which simply either improves or causes the deterioration of the laser cavity alignment, thereby effecting its output. The effect could not be correctly described as a laser oscillatory or pulsed operation as occurs in our invention. Further, their system, unlike ours, works better when operated near the threshold of stable laser action. Moreover, the intentional misalignment of the cavity output coupler which gives rise to the approximately 5 Hz oscillation in output of the instant invention is not taught by this reference. That is, the advantage of having a repetitive and highly reproducible laser signal which can be easily related to the sample absorptivity in that such signals are readily treated using standard detection and averaging techniques to achieve the maximum signal-to-noise ratio and therefore the greatest measurement sensitivity and range is not deducible from the Gordon et al. paper.
2. In "Time-Resolved Thermal Lens Calorimetry," by N. J. Dovichi and J. M. Harris, Anal. Chem. 53, 106 (1981), the authors describe a kinetic approach to measurements involving the thermal lens generation in weakly absorbing liquids. Therein it is explained that obtaining quantitative information from the time dependence of the signal derived from pulsing or chopping the heating laser is more efficient and reproducible than simply measuring the initial and final signal amplitudes alone with a long time delay in between these two sampling points. It is also pointed out that the signals obtained are more readily analyzed if one samples the output from the sample cell at short times since the thermal lens formed will then be relatively thin. No mention is made of intracavity insertion of the sample, with consequent oscillation of the laser output, however, as is described in the instant invention.
3. A more theoretical discussion of the time-resolved thermooptic phenomenon is found in "Photothermal Deflection Spectroscopy and Detection," by W. B. Jackson, N. M. Amer, A. C. Boccara, and D. Fournier, Appl. Optics 20, 1333 (1981). The authors describe the most common working embodiments of the thermooptic detection technique known in the art and show them schematically in their FIG. 4. As in Ref. 2, no mention is made of intracavity gain modulation by an absorbing medium under investigation.
References 1-3 then, provide no guidance for one skilled in the art to derive the method and apparatus of the instant invention. In particular, the observed laser oscillation and the relationship between the pulsewidth and the intracavity sample absorptivity represent a new thermooptic phenomenon based on the well-known negative thermal lens formation in laser heated materials.
4. The instant inventors have published a brief abstract which is not an enabling disclosure. In "Measurement of Weak Optical Absorptions by Thermally Induced Laser Pulsing," by David A. Cremers and Richard A. Keller, Conference on Lasers and Electro-optics Advance Program, Washington, D.C., June 10-12, 1981 distributed sometime in April, 1981, the authors mention pulsed laser operation when a weakly absorbing solution is introduced into a misaligned continuous-wave dye laser cavity. However, the key optical component which must be misaligned, and in what manner is not described. For example, a misalignment of the output coupler of the dye laser cavity in a horizontal direction when the folding mirrors are vertically situated will not produce pulsed operation, nor will any misalignment of the sample containing intracavity cell.